highly reflective coatings for interior and exterior steel cladding and the energy efficiency of...

Applied Energy 88 (2011) 4655–4666

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Applied Energy

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Highly reflective coatings for interior and exterior steel cladding and theenergy efficiency of buildings

Ali Joudi a, Harald Svedung b,⇑, Chris Bales a, Mats Rönnelid a

a Energy and Environmental Technology, Dalarna University, SE – 791 88 Falun, Swedenb SSAB EMEA, SE – 781 84 Borlänge, Sweden

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 March 2011Received in revised form 26 May 2011Accepted 1 June 2011Available online 6 July 2011

Keywords:Total solar reflectivityReflective coatingThermal emissivityBuilding interior heat fluxEnergy efficient buildingsEnergy saving

0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.06.002

⇑ Corresponding author. Tel.: +46 243 72376; fax: +E-mail address: [email protected] (H. Sved

The effect of surface heat-radiation properties of coil-coated steel cladding material on the energy effi-ciency of buildings in Nordic climate is addressed by parallel temperature and energy usage measure-ments in a series of test cabins with different exterior solar reflectivity and interior thermalreflectivity. During one year, a number of one- or two-week experiments with air conditioner coolingand electrical floor heating were made while logging air-, radiation- and surface temperatures, energyconsumption and weather conditions. Measurements show significant energy savings in the test cabinsby the use of high thermal reflectivity interior surfaces both during heating and cooling and a stronglyreduced cooling demand by the use of high solar reflectivity exterior surfaces. Results are interpretedwithin the context of a steady-state energy flux model, to illuminate the importance of surface resistanceproperties (radiation and convective heat dissipation).

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The resources used to maintain suitable interior thermal condi-tions in buildings involves a large part of the total energy turn-overin human activities [1,2] and the energy efficiency of buildings orlarger city scale energy systems depend on a great variety of phys-ical, climatic, social and human factors [3–8]. This work focuses onthe physical properties of the building envelope and more specifi-cally on the solar- and thermal optical properties of the interior aswell as exterior surfaces of the building envelope and how theseproperties may influence the detailed energy flux.

The use of high solar reflectivity and high thermal emissivity ofexterior building surfaces to reduce solar gain is becoming widelyunderstood and implemented through codes and regulations glob-ally [9]. With such ‘‘Cool Roof’’-properties, the amount of energyused for cooling the building and consequently the related CO2

emissions can be greatly reduced wherever there is a substantialcooling need during the larger part of the year [10–16]. In coldertempered climate and with limited internal heat-load there is, onthe other hand, a heating demand. This heating demand, althoughlarger during the dark winter season, will typically increase withincreased solar reflectivity and thermal emissivity of the exteriorsurfaces.

ll rights reserved.

46 243 71670.ung).

As the conductive heat transfer through the envelope of a build-ing depends largely on the interior and exterior surface tempera-tures as well as the total level of insulation [17], we aim to showhow the heat flux depends also on the thermal emissivity of theinterior building envelope surfaces. By the use of interior surfaceswith high thermal reflectivity, i.e., low emissivity in the thermallong wave IR-spectra, radiation heat flux between interior surfacesat different temperatures can be greatly reduced. Thus the buildingenvelope interior surface temperature will potentially be higherduring cooling and lower during heating, compared to a case withhighly emissive interior surfaces. This effect will be more pro-nounced in cases with radiative heating and cooling, i.e., withlow interior convection heat transfer as can be shown by a simplesteady-state model. Little work has been shown until now on thedependence of building energy flux on the interior surface thermalemissivity. In the work of Daoud et al. [18], the refrigeration load inice rinks are simulated for different ceiling surface emissivities in amodel containing radiation, zonal air flow and humidity transport.There, the thermal air stratification results are compared to mea-sured data, the radiation heat flux is shown to be dominant by afactor of 10 over the conductive and condensation heat fluxeswhen the ceiling emissivity is high and a very significant reductionin the total heat flux is shown for a low emissivity ceiling surface. Itcan be noted that an ice arena has a highly anisotropic temperaturedistribution compared to most other building applications.

To experimentally investigate and demonstrate how theamount of energy used for heating and cooling depends on the

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Fig. 1. The test cabins in Borlänge, Sweden (Latitude 60�N).

4656 A. Joudi et al. / Applied Energy 88 (2011) 4655–4666

total solar reflectivity and thermal emissivity on exterior surfacesand on the thermal reflectivity on interior surfaces in Nordic cli-mate, small test buildings, shown in Fig. 1, were built in Borlänge,Sweden, situated at latitude 60.4�N.

This work describes the optical properties of the different sur-face materials used as well as the design of the test cabins andthe measurement setup. Measurement results in terms of energyuse as well as radiation, surface and air temperatures are giventogether with measured climate data and interpreted within thetheoretical concepts of building energy flux. To highlight theseconcepts the dependence of the heat flux to the surface opticalproperties is presented in a simplistic steady state model. Usingthis model we also roughly estimate the corresponding depen-dence of the maximum exterior surface temperatures and solarheat gain in warmer climates. An inclusion of thermal massesand an evaluation of this model in dynamic building energy simu-lations are outside the scoop of the current work.

Table 1Building and HVAC system specification is given with a schematic view.

Location Borlänge, SE

Latitude 60� NorthWall U-value 0.247 W/m2 �CRoof U-value 0.207 W/m2 �CFloor U-value 0.158 W/m2 �CInside floor area 13 m2

Glazing Total 0.68 m2 double glazingCooling system Air conditioner EER:4.08Heating system Floor heating electricalVentilation system 70 m3/h nominal capacityInfiltration NegligibleInternal load Negligible

2. Test cabins

The test cabins, specified in Table 1, were built with coil-coated steel roof and wall claddings with different optical prop-erties. The three cabins used in this work are: (1) the ‘‘both-sidereflective cabin’’, having an interior surface coating with highthermal infrared reflectivity and an exterior surface coatingswith high solar reflectivity and high thermal emissivity, (2) the‘‘interior reflective cabin’’ having an interior surface coating withhigh thermal reflectivity and an exterior surface coating withlow solar reflectivity and high thermal emissivity and (3) the‘‘normal cabin’’ having an interior surface coating with low ther-mal reflectivity and an exterior surface coating with low solarreflectivity and high thermal emissivity. The optical propertiesof the interior and exterior wall and roof surfaces are given inTable 2, for the three cabins. The total solar reflectance, TSR,and the thermal emissivities are obtained as mean values ofthe measured reflectivity spectra using as weight factors the ter-restrial solar intensity and the 20 �C thermal infrared black bodydistributions, respectively. Solar and thermal reflectance spectrawere obtained using spectrophotometers equipped with whiteor gold integrating spheres, respectively. The interior floor sur-faces are identical in all cabins with a highly emissive PVC car-pet with a thermal emissivity of 0.9.

All exterior coatings have the same dark grey visible color. Inthe high TSR coating the carbon black pigment is replaced by an or-ganic black pigment with an extremely sharp decrease in absorp-tion going from the visual to the near infrared wavelengthswhere the solar radiation intensity is high. In the far infrared,above 2.5 lm, all exterior coatings show a high thermal absorp-tion. Exterior coatings are either textured polyester-melaminebased thermo-set coatings as on the interior reflective and bothsides reflective cabins or a thermoplastic PVC plastisol coating ason the normal cabin. Given the TSR and thermal emittance, the dif-ference in binder does not influence the thermal properties of thesurface materials significantly. The high thermal reflectivity inte-rior coating used in the interior reflective and both sides reflectivecabins is an epoxy based thermo-set coating with leafing Al-flakesdeveloped with the pure purpose of obtaining a thermal emissivityas low as possible. All coatings are applied and thermally cured onhot-dip galvanized steel substrate in an industrial coil-coating line.The coated steel sheet material is then roll-formed into LTP20 pro-file and cut to suitable length for exterior and interior cladding. Theoutdoor durability of the exterior material is still continuouslymonitored. The reduction of TSR has been shown to be small evenin marine environment but potentially sensitive to surface contam-ination. The use of leafing Al-flakes that tends to sit on top of the

Table 2Exterior and interior optical properties of the test cabins.

Both-side reflective cabin Exterior TSR = 0.39, IR-e = 0.92 (dark grey, CoolRoof)Interior IR-e = 0.25 (silver metallic)

Interior reflective cabin Exterior TSR = 0.10, IR-e = 0.92 (dark grey)Interior IR-e = 0.25 (silver metallic)

Normal cabin (reference case) Exterior TSR = 0.10, IR-e = 0.92 (dark grey)Interior IR-e = 0.91 (dark grey)

TSR: total solar reflectance.IR-e: thermal emissivity.

A. Joudi et al. / Applied Energy 88 (2011) 4655–4666 4657

paint surface increases the sensitivity to flake corrosion duringcontinuous condensation, showing a simultaneous visible degrada-tion and increase in thermal emissivity.

The roof and walls of the cabins are intermediately well insu-lated. The floor construction was relatively well insulated as shownin Table 1. The heating, ventilation and air-conditioning system,HVAC, includes electrical floor heating with 1 kW nominal powerand an air-to-air heat pump with a nominal energy efficiency ratioof 4.08 for cooling. Forced ventilation, used during heating experi-ments, includes an adjustable air inlet at feet level and an exhaustfan at top level with 70 m3 h�1 nominal capacity and an automaticshutter that closes when the fan is turned off.

3. Measurement procedure and instrumentation

Passive tests – without any heating, cooling or ventilation –were made during certain time periods in order to study the differ-ences, between the cabins, in response of the measured surface, air,and radiation temperatures to the daily variations of the externalconditions. Active tests, with the use of the HVAC system with cer-tain set-point temperatures, were made in order to compare, be-tween the cabins, the amount of energy needed for heating andcooling.

Along with the continuous measurement of power consump-tion, about 20 different air, surface and radiation temperatures ineach cabin were measured each second. Time averages werelogged each minute and analyzed on a weekly basis for a periodof one year.

Measurement instrumentation includes one PC-logger 3100from INTAB in each building. The logger has 24 programmableanalog inputs for thermocouples and voltages and six pulsecounters. The cold junction accuracy at 25 ± 10 �C is ±0.5 �C,the voltage accuracy is ±0.1% and the temperature coefficient ismax 50 ppm/�C. The software EasyView Pro was used for loggerset-up and data presentation. Infrared thermocouples, IRt/c 03-K™, from Exergen Corporation was used to measure directedradiation temperatures inside the test cabins. These devises havean interchangeability of ±1% according to the producer and areset to an emissivity of 0.9. Black sphere equipped K-type ther-mocouples were used to measure the operative temperature atchest level (1.5 m above the floor) and underneath the roof ridge(3.0 m above the floor). These globe thermometers measureapproximately the operative temperature, OPT [19]. This maybe used to calculate the mean radiation temperature, MRT. TheMRT for a sedentary person can be approximated using Eq. (1)from [20];

MRT ¼ T4g þ

1:10� 108V0:6a

eD0:4 ðTg � TaÞ" #1=4

ð1Þ

where Tg and Ta represents absolute globe and air temperatures,respectively, Va is the air velocity in ms�1, D is the globe diameter

in m and e is the globe thermometer emissivity which is 0.95 forblack globe. Exposed K-type thermocouples were used to measureinterior air-temperature at three levels above the floor as well asthe exterior ambient air-temperature. Surface temperatures weremeasured using surface mounted K-type thermocouples. The cali-bration coefficients provided by the manufacturer for the separatethermocouples were programmed into the loggers. To measurethe exterior global radiation (solar radiation), a GSM 3.3 Pyranom-eter from Adolf Thies GmbH & Co was used. This device has a spec-tral range from 300 to 5000 nm and an absolute error of less than10%. A rotating wind transmitter with a measuring range of 0.8–40 ms�1 mounted directly on top of a test cabin was used to mea-sure the air speed close to the cabins. Electricity consumption forheating, cooling and auxiliary use were measured separately usingthree separate pulse-generating electricity meters, ABB Cewe MiniMeter EE22 R, in each cabin. The nominal accuracy of these devisesis ±2%.

4. Steady-state model

To put the experimental investigation within a theoreticalframework, a very simple 2-node model of the building envelopeis used with interior and exterior boundaries. The correspondingthermal circuit of this one-dimensional steady-state heat transfermodel is given in Fig. 2. The two nodes represent the exteriorand interior roof or wall surfaces.

The corresponding heat flux equations include the conductionthrough the insulation, the exterior solar absorption, convectiveand irradiative interaction with the exterior surroundings andthe interior convective and irradiative heat transfer mechanisms:

Tso�TsiRcond

¼ Gð1� TSRÞ � Tso�TambRconv ;out

� re0ðT4so � T4

skyÞTso�Tsi

Rcond¼ Tsi�Tin

Rconv ;inþ reðT4

si � T4riÞ

8<: ð2Þ

where Tamp, Tin, Tsky, Tso, Tsi and Tri are ambient temperature, in-doorair temperature (adjacent to interior surface), sky temperature [21],exterior surface temperature, interior surface temperature and inte-rior radiation temperature (from interior surroundings as seen bythe interior surface), respectively in Kelvin. G is the incident solarirradiance on the exterior surface in Wm�2 and r is the Stefan–Boltzman constant. TSR, e0 and ei are total solar reflectance, thermalinfrared emissivity of the exterior and interior surfaces, respec-tively. Rcond. is the conductive thermal resistance in m2 K/W be-tween the exterior and interior surfaces. Rconv,out is the exteriorconvective thermal resistance, based on the wind velocity andRconv,in is the interior convective thermal resistance based on thedirection of the heat flux, according to [22]. The model is used tocalculate the exterior and interior surface temperatures and thenthe momentary heat flux through the building envelope undergiven input conditions. To numerically evaluate the model, the heatflux equations above are solved using the Solver function in Excel.

Fig. 2. Circuit diagram of the simplistic 2-node steady-state model including the surface temperature nodes, the exterior and interior boundary conditions and the heattransfer mechanisms.


5. Results

5.1. Experimental measurements

To clearly see how the daily temperature fluctuations in thecabins depend on the surface radiation properties, a passive testwithout heating or cooling and with the ventilation closed wasmade during a period in late June with long days and peak globalsolar irradiation close to 1 kW m�2. The global solar irradiationand ambient air temperature during the test are given as functionsof time in Fig. 3.

During this test, the day-time mean radiation temperature mea-sured at +3.0 m elevation is about 2� lower in the both-side reflec-tive cabin compared to that in the normal cabin as can be seen inFig. 4. The mean radiation temperature minima measured duringearly mornings as well as the day time peak values are less pro-nounced in the interior reflective cabin compared to the normalcabin.

Fig. 3. Global solar irradiation and ambient air tempe

Maximum exterior and interior surface temperatures measuredduring the passive test are given in Tables 3 and 4, respectively.The interior reflective cabin is showing higher values of the interiorroof surface temperature during hot days compared to the normalcabin, although the mean radiation temperature inside is lower. Aneven larger reduction in temperature gradient across the insulationin the both-side reflective cabin is due to the reduction in extremeexterior surface temperature due to the high TSR exterior coating.

To compare the vertical air temperature gradients in the differ-ent test cabins, the inside air temperatures have been measured atthree levels, i.e., +0.25, +1.5, +3.0 m above the floor. The differencesbetween the highest level air temperature and the lowest level airtemperature are shown for all three cabins in Fig. 5, as measuredduring two consecutive days in late June. Clearly, the vertical airtemperature gradient is slightly larger in the interior reflective ca-bin than in the normal cabin. The maximum values of the interiorair temperatures measured during the same days are given in Table5. The lowest indoor air temperatures are generally found in theboth-sides reflective cabin.

rature measured during the passive test period.

Fig. 4. Mean radiation temperatures at +3.0 m above the floor during the passive test.

Table 3Maximum exterior surface temperature during 24–27.06.2007.

Southroof (�C)

Southwall (�C)

Westwall (�C)

Northwall (�C)

Eastwall (�C)

Both-sidereflectivecabins

57.9 47.7 46.3 29.1 50.4

Interiorreflectivecabin

69.4 55.7 51.5 31.8 62.6

Normal cabin 66.0 54.8 56.0 30.8 58.5

Table 4Maximum interior surface temperature during 24–27.06.2007.

Southroof(�C)

Southwall(�C)

Westwall(�C)

Northwall(�C)

Eastwall(�C)

Floor(�C)

Both-sidereflectivecabins

27.4 26.6 26.3 26.1 26.0 25.3

Interiorreflectivecabin

29.4 28.3 27.9 27.4 27.4 26.3

Normal cabin 28.7 28.4 28.2 28.2 27.9 26.9


By analyzing the interior roof and floor surface temperaturedata from the normal cabin and the interior reflective cabin, the ef-fect of the low emissivity interior surfaces on the vertical heat flux

Fig. 5. The difference between inside air temperatures at +3.

in the interior volume during this passive experiment can be seen.As seen in Fig. 6, there is generally a larger difference between the

0 m and +0.25 m above the floor during the passive test.

Table 5Maximum interior air temperatures measured during the passive test between 24–27.06.2007.

Max. inside air temperature

At +0. 25 m floor above (�C) At +1.5 m above floor (�C) At +3.0 m floor above (�C)

Both-side reflective cabin 25 26 26.5Interior reflective cabin 26 27.3 28.2Normal cabin 27 28 28.5

Fig. 6. Interior south-facing roof and floor temperatures during the passive test.


roof and floor surface temperatures in the interior reflective cabinthan in the normal cabin. There is a significant time-lag betweenthe interior roof and floor surface temperatures in both cases con-sistent with a finite interior heat flux and a well insulated thermalmass in the floor.

Fig. 7. Global solar irradiation and ambient air tempera

A cooling test with closed ventilation was done during a warmsummer week with the global solar irradiation and ambient airtemperature as given in Fig. 7. During the cooling test the accumu-lative electrical power consumption of the separate cabins aremeasured.

ture measured during the early August cooling test.

Fig. 8. The accumulated energy used by the AC-unit with set point of 19 �C during the early August cooling test.


At a set temperature of 19 �C during the second week, the differ-ence in air conditioner electricity consumption is about 11% asseen in Fig. 8. Interior surface temperatures of the south facing roofduring this cooling experiment are shown in Fig. 9. The interior-reflective cabin shows a considerably higher interior roof surfacetemperature compared the normal cabin, indicating a reduced ver-tical radiation heat transfer from the roof interior surfaces to thelower parts of the cabins. The interior roof surface temperatureof the both-side reflective cabin is again lower, consistent withits lower exterior solar absorption.

During cooling hours when the air-conditioning units are work-ing, variations with periods on the minute timescale was detectedin all air-temperatures measured in the cabins. These fluctuationsmay be due to, e.g., variable direction of cold air output and otheroscillatory behavior of the AC-units. Occasionally, especially during

Fig. 9. Interior south-facing roof temperature during th

hot days, these variations disappeared from the measured +3.0 mlevel air-temperature signal only in the interior reflective cabinwhile still present in the normal cabin. During those occasionsthere were also an approximately 3 �C increase in the +3.0 m levelair-temperature in the interior reflective cabin compared to that inthe normal cabin while the foot- and chest level air-temperaturesremained similar. This strongly indicates a volume of air wellabove the AC-unit, not being cooled as efficiently as the air volumebelow.

A floor heating experiment was made during a two-week periodin early February. The outdoor air temperature and global solarirradiation is given in Fig. 10, as functions of time for the durationof the experiment. The measured electricity consumption with thefloor heating set to a floor surface temperature of 26 �C indicatesthat some energy saving can be made by using interior reflective

e early August cooling test with set point of 19 �C.

Fig. 10. Global solar irradiation and ambient air temperature measured during floor heating test.


coating. The accumulated consumptions, during this two-weekheating test, are 233 kW h for the normal cabin, 226 kW h for theinterior reflective cabin and 228 kW h for the both-side reflectivecabin. It can be noted that the heating penalty for the use of exte-rior reflective coating appears to be minor consistent with therelatively weak solar radiation although no internal heat load ispresent.

Keeping similar floor surface temperatures during heatinghours in the cabins, the radiation temperature measured withthe radiation pyrometers pointing upwards to the upper half ofthe test cabins showed higher values in the interior reflective cabinthan in the normal cabin. This can be seen in Fig. 11. The interiorroof surface temperature measured with surface mounted thermo-couples, on the other hand, showed significantly lower values inthe interior reflective cabin compared to the normal cabin, asshown in Fig. 12. The corresponding difference in the temperature

Fig. 11. Direct radiation temperature from upper part of

gradients across the thermal insulation is consistent with themeasured energy saving by the high thermal reflectivity interiorsurfaces.

5.2. Model simulation results

By evaluating the simplistic steady-state model during summerand winter conditions with variable exterior TSR and interior ther-mal emissivity values and remaining parameters set to match theexperimental conditions, the results can in principle be mappedwithin the assumptions of the model to the full range of TSR-and thermal emissivity values ranging from zero to unity. Primar-ily, the heat-flux equations of the model are solved for the exteriorand interior roof-surface temperatures and the heat-flux is thencalculated as conducted through the roof insulation.

the test cabins interior during the floor heating test.

Fig. 12. Interior south-facing roof surface temperature during the floor heating test.


In Fig. 13, the heat flux between the exterior and interior nodesis given as functions of the exterior TSR-value for a summer caseand a winter case, where in both cases, Rcond. = 3.2 m2 K/W (corre-sponding to an 80 mm PIR-panel), Rconv,out = 0.05 m2 K/W�1 (corre-sponding to a wind speed of 4 m/s), the exterior IR emissivity are0.9 and the interior air and radiation temperature are both set to20 �C. For the summer and winter case, Rconv,in values are1.43 m2 K/W�1 and 0.2 m2 K/W�1, respectively. In the summercase, as shown in Fig. 14, the heat flux from the exterior to the inte-rior appears to be reduced linearly with increasing exterior TSR.Between the TSR-values found in the test cabins the model givesa heat flux reduction of approximately 30%, comparable to themeasured results. During the winter case there is a linear increasein the heat flux from the interior to the exterior with increasingexterior TSR. This day-time heating penalty is somewhat larger

Fig. 13. Steady-state heat flux as functions

than the total heating penalty during the winter heating experi-ment above including long nights without sunshine and some-times snow covered surfaces.

The dependence of the heat flux on the interior thermal emis-sivity appears not to be linear. Fig. 14 shows the heat flux fromthe exterior to the interior during a summer case as functions ofthe interior surface thermal emissivity for three values of the inte-rior radiation temperature. In this case, Rcond. = 3.2 m2 K/W�1 (cor-responding to an 80 mm PIR-panel), Rconv,out = 0.05 m2 K/W�1

(corresponding to a wind speed of 4 m/s) and exterior TSR and IRthermal emissivity are 0.5 and 0.9, respectively. The interior con-vective resistance Rconv,in is 1.43 m2 K/W�1. This value representsa small downward convective heat transfer. It can be noted thatthe heat transfer reaches a limit at high emissivities asymptoticallyand at low emissivities there is a sharp decrease.

of exterior total solar reflectance, TSR.

Fig. 14. Steady-state heat flux as functions of the interior surface thermal emissivity for a summer case.


In the winter night case there is a heat flux from the interior tothe exterior. With a large roof this means interior convection in theupwards direction. Fig. 15 shows the total heat flux as functions ofthe interior emissivity for three values of the interior radiationtemperature, where Rcond. = 3.2 m2 K/W�1, Rconv,out = 0.05 m2

K/W�1 and exterior TSR and IR thermal emissivity are 0.5 and0.9, respectively. The interior convective resistance Rconv,in is0.2 m2 K/W�1. This value represents an upward convective heattransfer. As shown in Fig. 15, with a higher interior radiation tem-perature during heating, the lower interior surface emissivity has astrong diminishing effect on the total heat flux.

By using, as input to our model, readily available climate data,i.e., maximum solar irradiation and maximum ambient tempera-ture for an array of locations, the simple steady state evaluationprovides rough estimates of the climatic impact on maximumexterior surface temperature and maximum heat flux into the

Fig. 15. Steady-state heat flux as functions of the interio

building depending on the surface optical properties. In Table 6,this is shown for two different exterior TSR values for Stockholmrelatively close to our test site, Madrid in the south of Europeand Riadh in the Middle East. It can be noted that the reductionin heat-flux in terms of W m�2 is quite independent on the climateparameters under the given conditions. The reductions in maxi-mum exterior surface temperatures increasing the TSR from 0.1to 0.4 are close to our measured results in Table 3.

6. Discussion and conclusion

The experimental results strongly indicate possible energysavings from the right choice of surface radiation properties ofbuilding surface materials. Both measurements and theoretical cal-culations indicate considerable reduction of cooling demand by

r surface thermal emissivity for a winter night case.

Table 6Calculated maximum exterior surface temperature and heat flux at three geographical locations at low and high total solar reflectivities.

Location Latitude (�N) Max. solar radiationa,b (W/m2) Max. ambient temperaturec (�C) Max. exterior surface temperature (�C) Max. heat flux (W/m2)

TSR = 0.1 TSR = 0.4 TSR = 0.1 TSR = 0.4

Stockholm 59.35 967 28.7 57.3 46.7 11.1 7.9Madrid 40.45 992 38.7 67.0 56.3 13.9 10.8Riyadh 24.93 1078 45.7 75.9 64.5 16.6 13.2

a SMHI weather station at Bromma airport in Stockholm.b ASHRAEIWEC Weather File for Madrid and Riyadh. 2001 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA, USA.c 2005 ASHRAE Handbook – Fundamentals (SI).


using high-TSR exterior claddings. The increased heating demandwithout an internal heat load as in this case, resulting from theuse of high-TSR exterior claddings is represented by the simplemodel and shown in the experimental results to be relatively smallduring our heating experiment, given the time of the year and thegeographical location of the test cabins.

A slight reduction in TSR, going from 0.39 to 0.35, was measuredfor the material used as exterior surface on the both sides reflectivecabin during outdoor exposure for 1 year at the Bohus–Malmöntesting site on the Swedish west coast. The small reduction inTSR of the exterior surfaces due to degradation or dirt pick-up dur-ing the measurements, implied by the out-door exposure results,has little significance on the qualitative comparisons of the cabins.In locations with a significant general dirt pick-up this mayhowever be detrimental to the solar reflectivity of the exposedsurfaces and the long-term performance of the building. The highlyreflective interior material, although shown to be sensitive tocondensation, did not show any visible degradation during themeasurements.

In the simple steady-state model the radiation heat-transfer be-tween the highly emissive heated floor and the remaining interiorsurfaces with variable emissivity is represented by allowing theinterior surface node to interact with a radiation temperature rep-resenting in large the floor surface temperature. The effect that wesee in the measurements on heating consumption and interior sur-face temperatures by the use of low emissivity interior surfaces iswell represented by this simple model even with a relatively highinterior convective coefficient representing the upward verticaldirection of heat transfer. The even larger relative reduction in heattransfer through the insulation going to low emissivity interiorsurfaces during cooling on hot days as seen in the measurementsis also qualitatively reproduced by the simple model with a lowervalue of the convective coefficient representing the downwardheat transfer direction.

The steady state calculations are showing a large dependence ofthe total heat flux to the interior radiation temperature, even moreso during cooling using the low convection parameter value com-pared to the heating case. Clearly, the effect on the total heat trans-fer of a reduced interior surface emissivity would be morepronounced in cases with, say, a floor surface at an extreme tem-perature, e.g., using floor heating or in the case of an in-door icearena, compared to cases with a more isotropic interior tempera-ture distribution, e.g., a well insulated passive house.

Here we have used a simplistic steady state model to interpretthe larger trends in our measurement results and to project thetrends onto a small array of climate conditions. There is a goodagreement between the measurements and calculated surfacetemperatures and heat flux during, e.g., mid-day steady state likeconditions and the dependence on climate parameters, i.e., solarradiation and ambient temperature is reasonable. Remaining dis-crepancy may be due to the dynamic effect of thermal massesand the orientation of the cabin surfaces towards the sun normallyaccounted for in building energy simulations and also to the de-

tailed convective heat flux still posing a considerable challenge toreadily available simulation tools. Large differences in the time-re-solved climate conditions for different locations are also likely toinfluence the variability of the total amount of energy used forheating and cooling with respect to the surface optical properties,complementary to the steady state trends seen here.

Acknowledgments

Special thanks go to Ulf Kock for his great expertise and work-manship proven during the instrumentation of the test cabins.Becker Industrial Coatings AB and Akzo Nobel Industrial FinishesAB are warmly acknowledged for their cooperation. Financial sup-port from the Knowledge Foundation for our continued work isalso acknowledged.

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